Loans and Leases and the Allowance for Credit Losses - Summary of Changes in Accretable Yield for Acquired Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Purchased Impaired [Member]
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	$ 77,624	$ 26,082	$ 76,518	$ 37,230
Interest income	(5,865)	(4,149)	(16,843)	(15,583)
Reclassifications from nonaccretable balance	47	129	12,131	415
Balance at end of period	71,806	22,062	71,806	22,062
Other Acquired [Member]
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	344,989	450,970	397,379	538,633
Interest income	(37,396)	(39,019)	(118,697)	(135,105)
Reclassifications from nonaccretable balance	769	9,673	27,792	10,448
Other	4,697	1,870	6,585	9,518
Balance at end of period	$ 313,059	$ 423,494	$ 313,059	$ 423,494